Thai police shot rubber bullets and used water cannon and tear gas against pro-democracy protesters in Bangkok on Sunday at a rally outside a military barracks housing the prime minister's residence.
A youth-led protest movement calling for the resignation of Prayut Chan-o-cha's government had lost steam in recent months following a second wave of coronavirus infections in Thailand.
But the recent detention of four prominent protest leaders on royal defamation charges has given it a shot in the arm.
The four are among 58 protesters facing lese majeste charges and the prospect of up to 15 years in jail per charge if convicted of insulting the monarchy.
An estimated 2,000 demonstrators marched from the major Bangkok intersection Victory Monument on Sunday to a nearby military barracks where Prayut lives.
Among the protesters were scores of migrant workers from neighbouring Myanmar rallying against the coup that ousted civilian leader Aung San Suu Kyi on 1 February.
Some Thai protesters marched wearing hard hats and carried red flags.
Hundreds later pushed through shipping containers and barbed-wire barricades, leading to a confrontation with riot police guarding the entrance of the barracks.
Scuffles broke out between riot squad officers and protesters, while some demonstrators were seen pushing a police truck.
"They are preparing everything, shields, baton, water with some chemical and rubber bullets," one protester at the front line told Thai media.
Amid the tense stand-off, officers used water cannon trucks and let off tear gas in an attempt to disperse the crowd - the first use of force, albeit non-lethal, at a Bangkok rally for several months.
Protesters shouted for water and saline as they were sprayed. Some wore raincoats while others held their bare hands in the air or raised three figures - a symbol of resistance.
Officers later shot rubber bullets, according to a journalist on the ground.
"It doesn't hurt too much," a protester told Thai media, showing a red mark on his arm
Some protesters threw glass bottles, rocks and bricks at officers while others took refuge inside a nearby Shell petrol station, which later closed early.
Despite protest organisers sending a social media message at around 8:30 pm local time (1:30 pm GMT) urging demonstrators to go home, scores stuck around, many on motorbikes playing a cat and mouse game with advancing police.
A Facebook live stream showed footage of a damaged and graffitied police motorbike and car, as well as protesters trying to set fire to traffic cones and lighting fires on the road.
Erawan Emergency Medical Centre said 16 protesters were injured and a riot squad police officer lost consciousness.
A nurse at Rajavithi Hospital told reporters that the officer died but would not elaborate on the circumstances.
At least two protesters were arrested, according to the live Facebook feed, however police have not confirmed the total number.
Thai Lawyers for Human Rights said 19 were arrested including a 16-year-old.
A small group of protesters continued to clash with officers outside Din Daeng Police Station after midnight, into the wee hours of Monday morning.
Thailand's pro-democracy movement kicked off last July and at its peak attracted tens of thousands of mostly young people.
Among the movement's demands are a rewrite of the army-drafted constitution and reforms to the monarchy - a taboo-smashing demand in a country where the ultra-wealthy royal family has long been untouchable. - AFP